/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.tomcat.util.http.fileupload;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * <p>
 * An output stream which will retain data in memory until a specified threshold
 * is reached, and only then commit it to disk. If the stream is closed before
 * the threshold is reached, the data will not be written to disk at all.
 * </p>
 * 
 * @author <a href="mailto:martinc@apache.org">Martin Cooper</a>
 * 
 * @version $Id: DeferredFileOutputStream.java 467222 2006-10-24 03:17:11Z markt $
 */
public class DeferredFileOutputStream extends ThresholdingOutputStream {

	// ----------------------------------------------------------- Data members

	/**
	 * The output stream to which data will be written prior to the theshold
	 * being reached.
	 */
	private ByteArrayOutputStream memoryOutputStream;

	/**
	 * The output stream to which data will be written after the theshold is
	 * reached.
	 */
	private FileOutputStream diskOutputStream;

	/**
	 * The output stream to which data will be written at any given time. This
	 * will always be one of <code>memoryOutputStream</code> or
	 * <code>diskOutputStream</code>.
	 */
	private OutputStream currentOutputStream;

	/**
	 * The file to which output will be directed if the threshold is exceeded.
	 */
	private File outputFile;

	// ----------------------------------------------------------- Constructors

	/**
	 * Constructs an instance of this class which will trigger an event at the
	 * specified threshold, and save data to a file beyond that point.
	 * 
	 * @param threshold
	 *            The number of bytes at which to trigger an event.
	 * @param outputFile
	 *            The file to which data is saved beyond the threshold.
	 */
	public DeferredFileOutputStream(int threshold, File outputFile) {
		super(threshold);
		this.outputFile = outputFile;

		memoryOutputStream = new ByteArrayOutputStream(threshold);
		currentOutputStream = memoryOutputStream;
	}

	// --------------------------------------- ThresholdingOutputStream methods

	/**
	 * Returns the current output stream. This may be memory based or disk
	 * based, depending on the current state with respect to the threshold.
	 * 
	 * @return The underlying output stream.
	 * 
	 * @exception IOException
	 *                if an error occurs.
	 */
	protected OutputStream getStream() throws IOException {
		return currentOutputStream;
	}

	/**
	 * Switches the underlying output stream from a memory based stream to one
	 * that is backed by disk. This is the point at which we realise that too
	 * much data is being written to keep in memory, so we elect to switch to
	 * disk-based storage.
	 * 
	 * @exception IOException
	 *                if an error occurs.
	 */
	protected void thresholdReached() throws IOException {
		byte[] data = memoryOutputStream.toByteArray();
		FileOutputStream fos = new FileOutputStream(outputFile);
		fos.write(data);
		diskOutputStream = fos;
		currentOutputStream = fos;
		memoryOutputStream = null;
	}

	// --------------------------------------------------------- Public methods

	/**
	 * Determines whether or not the data for this output stream has been
	 * retained in memory.
	 * 
	 * @return <code>true</code> if the data is available in memory;
	 *         <code>false</code> otherwise.
	 */
	public boolean isInMemory() {
		return (!isThresholdExceeded());
	}

	/**
	 * Returns the data for this output stream as an array of bytes, assuming
	 * that the data has been retained in memory. If the data was written to
	 * disk, this method returns <code>null</code>.
	 * 
	 * @return The data for this output stream, or <code>null</code> if no
	 *         such data is available.
	 */
	public byte[] getData() {
		if (memoryOutputStream != null) {
			return memoryOutputStream.toByteArray();
		}
		return null;
	}

	/**
	 * Returns the data for this output stream as a <code>File</code>,
	 * assuming that the data was written to disk. If the data was retained in
	 * memory, this method returns <code>null</code>.
	 * 
	 * @return The file for this output stream, or <code>null</code> if no
	 *         such file exists.
	 */
	public File getFile() {
		return outputFile;
	}
}
